As well as Sassoon's own Memoirs of an Infantry Officer can I recommend Robert Graves' Goodbye to All That which is IMOH one of the best works written about the war. Sassoon and Graves were fellow officers in the Royal Welsh Fusiliers, and Graves was greatly concerned at what might happen to Sassoon because of his action, and visited him at Craiglockhart Hospital to attempt to advise him on what he should do.
In addition to these books, Pat Barker's Regeneration, the first in her Great War trilogy describes Sassoon's stay at Craiglockhart in detail, in particular the relationship between him and the army psychologist who treated him, Captain W.H.R.Rivers - fascinating stuff.
Some of the material which has been saved was on display at The Imperial War Museum a few years back in the War Poets exhibition - great to know that others will be able to see and be inspired by it in the future.